You have a somewhat difficult problem here - the tenant's complaint is one of "habitability" (the tenant has a right to a working sewer independent of the landlord's right to payment of rent).
However, if your unlawful detainer is based on multiple breaches (non-payment of rent, non-permitted tenants, and damage to the property (including most importantly, damage to the sewer in question), a court may be willing to grant you your unlawful detainer judgment in any event.
(This really is a "close" case (any time the tenant raises a viable habitability argument, it becomes an issue the landlord should take very seriously), and I would highly encourage you to retain a lawyer to represent you as soon as possible (a landlord/tenant attorney could step in and help you with this before Thursday).
I do know that customers do not come to this site to be told "go hire a lawyer" but if you lose your case on a habitability claim, not only will you be forced to make the repairs, but you are going to find yourself stuck with these tenants for a lot longer (they can raise this as a defense for the next six months, claiming that future attempts to evict them are "retaliatory"). Frankly, you probably have at least a reasonable case (if not a good one), but it must be handled carefully as the repercussions can be significant!